1. Field of the Invention
The present invention is directed generally to a rear axle assembly for a wheelchair and more particularly to a rear axle assembly for a wheelchair that provides for vertical height adjustment of the axle of the rear wheel relative to the frame of the wheelchair.
2. Description of the Related Art
A typical wheelchair includes a seat supported by a frame resting upon four wheels. A front wheel and a rear wheel are disposed on each side of the wheelchair. The pair of rear wheels may be located under or behind the user. The front wheels are usually located in front of the user and pivot to facilitate steering. Generally, the rear wheels are mounted to opposite sides of the frame by an axle assembly. The front wheels may each be mounted to opposite sides of the frame by a bearing fork assembly.
A wheelchair may require adjustment to fit a particular user. For example, many wheelchairs allow the user to adjust the height of the seat at the rear of the wheelchair, and therefore the angle of the seat, by moving the position of the rear axles vertically relative to the frame. The ability to change the angle of the seat is important to proper positioning of the wheelchair occupant. Increasing the angle of the seat (i.e., increasing the height of the front of the seat relative to the back of the seat) allows gravity to help prevent the occupant of the wheelchair, who will likely have limited or no lower extremity function and perhaps limited trunk muscular control, from sliding out of the seat. Often, finding the correct angle of the seat is a matter of trial and error. Over time, the most desirable angle may change. For example, a particular wheelchair user may have a degenerative disease that requires increasing the angle over time. Alternatively, as people age they become more susceptible to pressure sores. By reducing the angle of the seat, it is possible to reduce the pressure on the buttocks.
Similarly, many wheelchair designs allow a user to move the rear wheels longitudinally forward and backward relative to the seat. In this manner, the rear wheels may be positioned relative to the load they bear. To self propel a wheelchair, the occupant may apply a rotational force directly to the rear wheels. Typically, each of the rear wheels of a wheelchair will include a handrim. The occupant of the wheelchair may apply a rotational force directly to each of the rear wheels by grasping each of the handrims and applying a rotational force thereto. The amount of rotational force that must be applied to self propel the wheelchair can be reduced by positioning the rear wheels longitudinally so that the axles (which are at the centers of the rear wheels) are located as close as possible to the center of gravity of the occupied wheelchair. However, as a pivot point defined by the axles is positioned closer and closer to the center of gravity, the likelihood of the chair tipping over backwards increases, thereby rendering the chair more unstable. Therefore, it may be desirable for a user to select a longitudinal position of the rear wheels that gives the user the best tradeoff between ease of rear wheel rotation and stability.
The tradeoff selected by a particular user may be influenced by their experience level. For example, a novice unfamiliar with operating a wheelchair may prefer increased stability. However, an expert at operating the wheelchair may have little difficulty with a decreased level of stability. As a user becomes more expert at operating a wheelchair, he/she may reposition the rear wheels to achieve a more desirable tradeoff.
Consequently, the axle assemblies included with many wheelchair designs allow for vertical and/or longitudinal adjustments. In prior art designs, the vertical adjustment is typically achieved by drilling multiple through-holes into two vertical tubular frame members. A horizontal axle plate may then be fastened to the frame at the location of these through-holes by pins or other fasteners extending through one of the through-holes. The rear wheels are fastened to the axle plates, which typically have multiple through-holes to permit longitudinal adjustment of the rear axle position as well as the vertical adjustment provided by the through-holes in the vertical tubular frame members.
This method has several drawbacks. First, the vertical frame members are weakened by the through-holes drilled therein. In some designs, an additional vertical member is required to help bear the load. The additional vertical member may increase the weight of the chair. Second, drilling the through-holes is an additional processing step that adds expense to the manufacture of the wheelchair. Third, the load bearing requirements placed on the vertical frame members limits the minimum distance between the through-holes. If the through-holes are too close together, the material between the through-holes may not support the load of the wheelchair transferred to the rear wheel axles. This limitation may determine the number of possible height adjustment options available to a user. Finally, the through-holes are generally visible to the user and detract from the visual appeal of the wheelchair.
In another prior art design, the vertical adjustment is typically achieved by fastening a vertically oriented axle plate to the frame. The axle plate has multiple through-holes that permit the rear wheels to be fastened to the axle plate at varying heights. The vertical axle plate can be repositioned longitudinally along the frame thereby providing horizontal adjustment of the rear axle position as well as the vertical adjustment provided by the through-holes in the axle plate.
This method also has several drawbacks. First, the vertical axle plates are weakened by the through-holes drilled therein. Second, drilling the through-holes is an additional processing step that adds expense to the manufacture of the wheelchair. Third, the load bearing requirements placed on the vertical axle plates limits the minimum distance between the through-holes. If the through-holes are too close together, the material between the through-holes may not support the load of the wheelchair transferred to the rear wheel axles. This limitation may determine the number of possible height adjustment options available to a user. Finally, the through-holes are generally visible to the user and detract from the visual appeal of the wheelchair.
Therefore, a need exists for wheelchair designs with more rear seat height adjustment options. A further need exists for a rear wheel mounting assembly that allows for height adjustment without the use of through-holes drilled through load-bearing frame members or vertical axle plates.